`std::sys` configurability

[mkroening]

In #113358, I tried to add a check for poll returning ENOSYS to the runtime initialization path of Linux.

The use-case was adding a target for the Unikraft Unikernel Development Kit, which does not support poll, in its most basic configuration and instead stubs it, returning ENOSYS.

@workingjubilee proposed in #113358 (comment) that we might want to make sys easier to configure according to different target's capabilities (even from target-spec.json).

@rustbot label: +A-technical-debt

[mkroening]

I just opened #113411, where I added a Unikraft-specific cfg.